1. Field of the Invention
The present invention relates to a digital still camera having an electronic shutter function, and more particularly to a solid-state image capturing device capable of acquiring a captured image which is not influenced by a smear when releasing a high-speed shutter to capturing an image by the electronic shutter function, a smear charge removing method and a digital still camera.
2. Description of the Related Art
FIG. 18 is a schematic plan view showing a conventional solid-state image capturing device having pixels arranged like a square grid. In a solid-state image capturing device 10, a photoreceptor group includes a photoreceptor 11b having a blue filter attached thereto and a photoreceptor 11g having a green filter attached thereto which are alternately arranged in a transverse row and a photoreceptor group includes the photoreceptor 11g having the green filter attached thereto and a photoreceptor 11r having a red filter attached thereto which are alternately arranged in a transverse row, and both of the groups are alternately arranged in a vertical direction.
A transfer electrode 16 is provided around each of the photoreceptors 11r, 11g and 11b, and a transfer electrode group in a vertical column which is constituted by the transfer electrode 16 forms a vertical CCD register 12 (only one column is shown in a dotted line). A first transfer electrode 16 group in a transverse row which is arranged in the upper side portions of the photoreceptors 11r, 11g and 11b is connected to an electrode terminal 21, a second transfer electrode 16 group in a transverse row which is arranged in the lower side portions of the photoreceptors 11r, 11g and 11b is connected to an electrode terminal 22, and a third transfer electrode 16 group in a transverse row which is provided between a photoreceptor group arranged in a transverse column and a photoreceptor group arranged in a next transverse row is connected to an electrode terminal 23.
When a reading potential is applied to a reading gate which is not shown in order to read image signals picked up by the solid-state image capturing device 10 (the received charges of the photoreceptors 11r, 11g and 11b), the received charges (signal charges) of the photoreceptors 11r, 11g and 11b are read onto the transfer electrode 16 of the vertical CCD register 12 as shown in an arrow on each of the photoreceptors 11r, 11g and 11b. 
A transfer potential (a transfer pulse) is sequentially applied to each of the electrode terminals 21 to 23 so that a received charge is transferred in a vertical direction (a downward direction in the example shown in the drawing) and a signal charge is transferred to a horizontal register 13 provided in the lowest stage. The signal charge is transferred in a horizontal direction by the application of the transfer pulse to electrode terminals 25 and 26 and is output from an output section 14 of the horizontal register 13. Moreover, the solid-state image capturing device 10 is provided with an electrode terminal 28 for OFD pulse application which will be described below.
FIG. 19 is a diagram showing the operation timing of a digital still camera mounting a mechanical shutter and the conventional solid-state image capturing device. In the case in which a shutter button is not released in the digital still camera, a captured image (a dynamic image) is displayed on the LCD display section at the back face of the camera to serve as a finder. Therefore, a vertical blanking pulse is generated at an interval of 1/30 to 1/60 second and a captured image signal is fetched every 1/30 to 1/60 second, for example.
When the shutter button is released (ON), a time interval between subsequent vertical blanking pulses is set to be 1/10 second, for example, and a static image is fetched with each of reading pulses B1, B2, . . . generated in vertical blanking pulses A1, A2, . . . .
However, the image signal fetched with the first reading pulse B1 is cancelled as a dummy output and the image signal fetched with the next reading pulse B2 is output as the image signals of a static image. Moreover, an OFD (overflow drain) pulse is applied after the first vertical blanking pulse A1, and the stored charges of the photoreceptors 11r, 11g and 11b formed on the surface of the semiconductor substrate of the solid-state image capturing device are drained toward the back side of the semiconductor substrate, and the stored charges of the photoreceptors 11r, 11g and 11b are zero cleared before the start of the image capturing.
Moreover, the transfer pulse (FIG. 19 shows only a second-phase transfer pulse VP2 and a change in an ON/OFF state is omitted in cross-hatching) which is applied from the electrode terminals 21, 22 and 23 to the vertical CCD register 12 is transmitted at a high speed immediately after the rise of the next vertical blanking pulse A2 till the rise of the reading pulse B2 (a blackened portion X in the drawing), and the electric charge in the vertical CCD register 12 is swept at a high speed before the signal charge of image information is fetched from the photoreceptors 11r, 11g and 11b into the vertical CCD register 12.
After the application of the OFD pulse till the closing of the mechanical shutter, the signal charges stored in the photoreceptors 11r, 11g and 11b are read and transferred to the vertical CCD register 12, and are output as the signals of a static image.
FIG. 20 is a timing chart for explaining the transfer state of the vertical CCD register 12 in the solid-state image capturing device 10 shown in FIG. 18. In the drawing, an encircled figure represents a last one digit of the designation of each of the electrode terminals 21 to 23. By cyclically applying a transfer potential (for example, pulses having a high level (0 V) and a low level (−8 V) to each electrode terminal, an electric potential well is moved along the vertical CCD register 12 and signal charges Qsiga, Qsigb, . . . constituting the image signals are transferred to the horizontal register 13.
At this time, light or an electron leaks from the photoreceptors 11r, 11g and 11b into the vertical CCD register 12 also before the received charges of pixels A, B, C . . . (the photoreceptors 11r, 11g and 11b) are read onto the vertical CCD register 12, and the original signal charges Qsiga, Qsigb, . . . , and furthermore, a smear charge qsmr causing a smear (a bright line in a longitudinal direction which appears in an image when the sun is photographed) enters each electric potential well.
As described above, a high-speed sweeping pulse X is applied to the vertical CCD register 12 in timings t8 to t9 of FIG. 19, and the smear charge qsmr is swept completely to cause each electric potential well to be completely empty (timing t9), and the signal charges Qsiga, Qsigb, . . . of the pixels A, B, . . . are then read (timing t10) and are transferred to the horizontal register 13. Consequently, it is possible to obtain an excellent image having no smear.
However, the operation timing of the digital still camera is illustrated in FIG. 19 as an example of a low-speed shutter using a mechanical shutter. While the vertical CCD register 12 is being driven by the sweeping pulse X, the mechanical shutter is maintained in a closing state. Therefore, there is no possibility that a new smear charge might enter the electric potential well.
On the other hand, in the case in which an image is to be picked up by a high-speed shutter, the mechanical shutter cannot release the high-speed shutter. As shown in FIG. 21, therefore, the high-speed shutter is released by the electronic shutter function. In this case, a period from the timing t6 immediately after the OFD pulse to the timing t8 of the reading pulse B2 is set to be a shutter time in a state in which the mechanical shutter is opened, and the signal charges stored in the photoreceptors 11r, 11g and 11b are read as the signals of captured images in the shutter time.
Also in this case, the smear charge in the vertical CCD register 12 is swept by the sweeping pulse X before the reading pulse B2 and the sweeping is carried out in a state in which the mechanical shutter is opened. Therefore, the smear charge qsmr enters the electric potential well during the sweeping.
In the case in which the high-speed shutter is released by the electronic shutter function, accordingly, smear charges qi+1, qi, . . . enter the electric potential well for transferring the signal charges Qsiga, Qsigb, . . . as shown in FIG. 22 so that the same smear charges are transferred together with the signal charges. The smear charges qi+1, qi, . . . cannot be distinguished from the signal charges Qsiga, Qsigb, . . . . For this reason, the picture quality of the captured image is deteriorated.